A process for obtaining direct positive images by processing an internal latent image-type silver halide photographic emulsion with a surface developer in the presence of a fogging agent, as well as silver halide photographic emulsions and light-sensitive materials for such a process, are known, as disclosed for example, in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, and 3,227,552, British Pat. Nos. 1,011,062 and 1,151,363, Japanese Patent Publication No. 29405/68, etc.
By the expression "internal latent image-type silver halide photographic emulsion" is meant a silver halide photographic emulsion which has photosensitive centers mainly inside of the silver halide grains, and a latent image is formed inside of the silver halide grains due to such photosensitive centers. A silver halide photographic emulsion composed of such silver halide grains is not substantially developed by a surface developer.
By the expression "surface developer" herein is meant a developer which develops a surface latent image formed on silver halide grains, but which does not substantially develop an internal latent image. Such a surface developer or surface developer composition typically contains a conventional silver halide developing agent, but must substantially not contain a silver halide solvent (e.g., a water-soluble thiocyanate, a water-soluble thiosulfate, ammonia, etc.) which is used for developing an internal latent image of silver halide grains. The surface developer may contain a small amount of excess halide, or such excess halide may be present in a silver halide emulsion layer in the form of a halide-releasing compound. However, the use of a large amount of a halide must usually be avoided, to prevent the occurrence of substantial decomposition or dissolution of silver halide grains.
In the above-noted process for obtaining direct positive images, the fogging agent may be present in a developer or in a silver halide photographic emulsion layer or other layer of a photographic material.
As a direct positive silver halide emulsion, a previously surface-fogged silver halide emulsion (i.e., a previously fogged-type direct positive silver halide emulsion) is well known in addition to an unfogged internal latent image-type direct positive silver halide emulsion which is developed in the presence of a fogging agent as described above. Also, it is well known that the silver halide emulsion of these previously fogged-type and unfogged-type can be spectrally sensitized by sensitizing dyes as described, for example, in U.S. Pat. No. 3,537,858, etc.
However, the process for forming images using an internal latent image-type direct positive silver halide emulsion (i.e., a direct positive silver halide emulsion which is surface-developed in the presence of a fogging agent) is utterly different from the process of forming images using a previously fogged-type direct positive silver halide emulsion. Thus, a sensitizing dye effective for the previously fogged direct silver halide emulsion is not always effective for the internal latent image-type direct positive silver halide emulsion. In the latter type silver halide emulsion, direct positive images are obtained by performing the surface development thereof in the presence of a fogging agent. In this case, there is an interaction between sensitizing dye and fogging agent, and the sensitizing dye frequently exhibits an action of accelerating or controlling the action of the fogging agent in addition to the essential sensitizing action.
This is described, for example, in Research Disclosure, No. 15162 (1976), page 78, lines 6 to 17, wherein it is noted that in the process for forming direct positive images using an internal latent image-type emulsion, negatively charged dyes, dyes having pair ions, and chargeless dyes are more effective than positively charged cyanine dyes, and among them, merocyanine dyes having a carboxy group as a substituent are particularly effective.
Also, in order to employ such process for obtaining direct positive images in the presence of a fogging agent for many photographic applications, improvement in the stability of the reversal photographic properties of the internal latent image-type direct positive silver halide emulsion layers with the passage of time during storage prior to development, and in particular the stability of the reversal photographic properties thereof in the case of storing them at high temperature and high humidity for a long period of time, has been desired.
Furthermore, if the developing temperature is high, there is a tendency that the change in photographic properties (sensitivity, D.sub.min, gradation, etc.) exhibited due to the passage of time is relatively large as compared to the case of low temperature development, and hence improvement with respect to this point has also been desired.